This invention relates generally to the presentation of characters on a video display, and particularly relates to an improved multi-function character generator particularly adapted for use in a television receiver.
A digital data display system typically receives digitally encoded information from a computer and provides a presentation of that information in the form of characters on the screen of a video display such as a cathode ray tube (CRT). The display of selected characters under computer control is generally accomplished by simultaneously varying the horizontal and vertical beam deflection signals to the CRT in synchronization with electron beam intensity control according to displayed character configuration. The deflection signals are usually current or voltages provided to the CRT deflection system while electron beam intensity is controlled by CRT inter-electrode potentials. The characters are formed as a sequence of discrete, intensified units of area in the form of a dot matrix in positions defined by the vertical and horizontal sweep voltages. The rapid vertical and horizontal sweeping of the CRT faceplate by the electron beam, the intensity of which is selectively modulated, generates the individual dots which comprise individual character configurations.
The computerized video display system generally includes a central processing unit for performing data processing tasks, such as the input and output of digital data signals, storing digital data in a memory, and selectively reading out this stored data and providing it to the CRT in generating the desired character array on the face of the CRT. Information may be provided to the computer by a variety of means, the most popular currently being a keyboard.
Interfacing the computer with the video display generally is a signal processing circuit termed the character generator. A typical character generator includes a read only memory (ROM) in which are stored digital codes for the dot matrix display of individual characters. Also included in the character generator is a random access memory (RAM) which includes various storage locations corresponding to video display faceplate positions in which are stored coded signals representing individual characters received from the computer. A controller in the character generator is responsible for selectively accessing the stored contents in the ROM in accordance with the coded signals stored in RAM and for providing these selectively read-out dot matrix arrangements to the video display. Essential to the operation of the character generator is a source of clocking signals for the proper timing of input and output signals in achieving synchronization with electron beam sweep and for establishing proper timing between the RAM and ROM and various other character generator components in carrying out essential signal processing operations.
Most recently, word processor systems utilized for document creation and text editing have become another new application for computer driven video displays. One such computer driven video display system is disclosed in U.S. Pat. No. 4,243,987 to Bobick wherein is described a video display processor which receives coded character data and digitally encoded text manipulative data from an input device such as a keyboard and generates horizontal sync, vertical sync, and video data in the form of a display scan line dot pattern. The display processor is comprised of a display refresh memory for retaining text character codes and text manipulative codes, a character generator that receives text and text manipulative data from the display refresh memory and converts it to a video signal, video output circuitry interfacing the display processor with a video display, and microprogram timing and control logic to provide the required timing and control signals for proper operation sequencing. This device, similar to other word processing systems, is designed to display a full page of type-written copy and thus lacks flexibility in limited size text positioning on the video display face and is incapable of performing functions in addition to those related to character display.
Another approach to digital symbol generation is disclosed in U.S. Pat. No. 3,713,135 to Lazecki which is primarily concerned with the positioning of displayed characters on the face of a CRT. In this system the vertical location of a character segment upon the display is completely described by a pair of dimension numbers, where one dimension number equals the raster line identification number of the topmost line of the segment and the other number equals the raster line identification number of the line below that segment. By performing subtraction operations involving segment lines located between these two numbers and segment lines located outside the vertical space defined by these two numbers, either a positive or negative number will result, from which the raster lines on the CRT where it is desired to display a given character may be determined. This approach allegedly simplifies and makes more reliable video display raster control. Another approach to more flexible character positioning on a video display is disclosed in U.S. Pat. No. 4,215,343 to Ejiri et al. which makes use of recirculating shift registers including respectively display pattern information and display position information in a computer driven CRT. The location within a shift register of a particular pattern code corresponds to the horizontal position where that pattern is to be displayed on the face of the CRT. The display position codes are indicative of the vertical positions of the corresponding patterns represented by the pattern codes. This digital pattern display system includes two ROMs one of which is primarily involved with character vertical positioning while the other ROM functions primarily in generating the proper character pattern to be displayed. The last two discussed patents function solely and exclusively as vertical positioning controllers for the proper location of characters on the face of the CRT.
A video display control system for use with a conventional television receiver is disclosed in U.S. Pat. No. 4,026,555 to Kirschner et al. This apparatus incorporates a random access memory (RAM) having a plurality of data storage positions for maintaining a digital representation of the data to be displayed on the television screen. Display data is written into the RAM under the control of a programmed microprocessor which modifies the display data stored in memory in accordance with manual keyboard entries. The display data is read from the memory in synchronism with the scanning of the television screen. In addition, the microprocessor is adapted to perform a variety of standard calculator functions to permit the television screen to display performed calculator operations. Thus, this system is designed to function solely as a video display controller and is not intended to perform additional functions in the television receiver environment.
The present invention, while fully capable of controlling a video display in the presentation of a great variety of character arrays, is also capable of providing additional control signals to the television receiver, or to any system in which it is used, for performing additional functions therein.